Generally, a controller for controlling a load installed in a vehicle is equipped with a switching circuit protector for immediately breaking a switching circuit when an overcurrent is caused in the load. PTL 1 discloses such a conventional switching circuit protector. A switching circuit protector according to the conventional example disclosed in PTL 1 is configured to estimate a temperature of a switching circuit by calculating a quantity of heat generation and a quantity of heat radiation of the switching circuit (including a cable connecting a load to a power supply, and a semiconductor switch) based on a current flowing in the load and measuring an ambient temperature, and break the switching circuit when the estimated temperature is equal to or higher than a threshold value thereby protecting the load connected to the switching circuit.
However, when the semiconductor switch is controlled with PWM (pulse width modulation) to operate the load, the switching circuit protector according to the conventional example may fail to appropriately break the semiconductor switch when the cable is heated if an error is caused in the estimated temperature. The following is an explanation of such a case with reference to the timing chart illustrated in FIG. 1.
In FIG. 1, plot (a) represents sampling period for estimating the temperature, plot (b) represents a timing of ON/OFF signals of the semiconductor switch with the PWM drive, and plot (c) represents a current flowing in the cable. The temperature of the cable repeats a cycle, as illustrated by a curve P11, where the temperature increases when on-duty and decreases when off-duty. In other words, the temperature of the cable increases when turning on the semiconductor switch, and decreases when turning off the semiconductor switch. Thus, the temperature of the cable varies to repeat the cycle of the ON/OFF state.
When an error is caused in the estimated temperature of the cable, the estimated temperature may indicate a numerical value lower than the actual temperature of the cable, as illustrated by a curve P12 in FIG. 1. That is, although the actual temperature of the cable (the curve P11) exceeds a threshold value Tth, the estimated temperature (the curve P12) may fall below the threshold value Tth. In such a case, the protector cannot break the semiconductor switch. As a result, the temperature of the cable may increase and then exceed the threshold temperature Tth.